Production of coated board for gloss ink printing and article produced thereby



tates PRODUCTION {EF CQ'A'I'ED BQARD FOR GLLSS INK PRINTENG AND ARTECLEPRODUCED THEREBY No Drawing. Application April 2%, 1953, Serial No.349,958

Our invention relates to that mode of producing coated paper orpaperboard (and to the type of product produced thereby) wherein thecharacteristic steps are (a) an initial plasticization of the websurface by passing it through a breaker stack of calender rolls whiletreating its surface with a plasticizing solution imposed thereon fromone or more water boxes on the rolls, (b) the irrprinting upon theplasticized surface of a suitable coating composition of mineral coatingsubstance and an adhesive, and (c) drying and calendering the coatedweb. Such a process is taught in Patent 2,419,207, dated April 22, 1947,to Harry C. Fisher, one of the applicants herein. In accordance with theteachings of this patent, the plasticizing solution may be a solution ofpolyvinyl alcohol or a solution of gelled starch, or similar materials,

or a mixture of such materials. By reason of the plasticization of thesurface of the paper or paperboard web, it becomes possible to make anexcellent coated product economically by imprinting thereon a very thinlayer of the coating composition. A coating composition taught in theaforesaid Fisher patent is a composition com prising suitable mineralcoating substances and starch as a binder. Very excellent coated boardshave been produced in this fashion having highly attractive coatedsurfaces well adapted for printing; and such boards have attained widecommercial success.

Procedures for making such coated boards are capable of modification invarious ways. By way of example, as taught in Patent 2,515,340, datedJuly 18, 1950, to Fisher and Sooy, it is possible to make such boardsresistant to wet rubbing by the addition of a resinous substance and acatalyst therefor, these substances being imposed upon the board inseparate increments. The catalyst may thus be applied as a constituentof the plasticizing solution, and the resin as a constituentof themineral coating composition," or vice versa. -As in the copendingapplication of Stanley W. Trosset, J rz, Harry C. Fisher and SimeonRichard Holmes, Serial No. 164,748, filed May 27, 1950, now U. 8. Patent2,698,259 and entitled Manufacture of Water Finished Board Having anInsolubilized Protein-Mineral Coating Thereon, similar procedures havebeen applied to the manufacture of coated paper or board in which theadhesive of the mineral coating is casein, a hardening agent for thecasein, such as alum, being added separately, as in the plasticizingsolution or in a separate operation by means of water boxes on afinishing calender stack, or at both places.

Coated papers and boards produced in this way have had the advantage ofsubstantial economy since very small quantities of the coating substanceare employed and, by reason of the pretreatment of the web surface, animprinting operation serves to give a good surface, renderingunnecessary such operations as smoothing, brushing, casting or the like,practiced upon a layer of wet coating mixture on the surface of theboard. Moreover, the coated products have had attractive, smooth, andgenerally matte surfaces which are excellent for printing both withgloss and non-gloss inks. Where gloss inks are employed, the contrastbetween the glossy surface of the printed impression and the mattesurface of the coated web where ink is absent has been found especiallyattractive.

It has been recognized, however, that for gloss ink printing thesurfaces of these coated papers or boards had under some circumstances agreater absorptivity than was desirable, leading to the use ofrelatively large quantities of gloss vehicle in the inks employed wherethe maximum gloss was desired in the printed impression. The presentinvention deals with the problem of diminishing the effect of thisabsorptivity without changing the appearance of the coated stock so thatthe glossy effect of gloss printing inks will be enhanced, and thecontrast between its appearance and the appearance of the unprintedcoated stock also enhanced. The utility of the coated papers or board ofthe present invention is not confined to gloss printing operations sincethe appearance of the printing with ordinary inks is also improved.

The solution of this problem is the primary object of our invention.This and other objects, which will be set forth hereinafter or will beapparent to one skilled in the art upon reading this specification, weaccomplish in that procedure and product of which we shall hereinafterdescribe certain exemplary embodiments.

In the manufacture of the improved product of our invention, it may bestated that we prefer to carry on the plasticization of the board insuch fashion that it is very thorough, using for the purpose anincreased amount of theplasticizing solution. It will be understood fromPatent 2,419,207 referred to above that the imprinting of the coatingoccurs during a persistence of'the plasticizing effect, which means inpart that when the surface of the paper or board has been mechanicallyworked by the action of the rolls of the breaker stack in the presenceof the plasticizing solution, the coating is imprinted thereon beforethe imposed plasticizing solution has thoroughly dried. We have foundthat the use of greater quantities of the plasticizing solution in theplasticizing step not only has the effect of rendering the paper orboard surface itself less absorptive, but also that the plasticizingsolution tends in some measure to permeate the imprinted coating,diminishing its absorptivity to auseful extent.

It would ordinarily be thought that the problem of diminishing theabsorptivity of a mineral coating on paper or board could be solved in asimple'fashion merely by increasing the adheslvezmineral ratio in thesuspension or slurry applied to the web for coating purposes. it must berecalled, however, that in a process such as has been described theconsistency and characteristics of the mineral coating mixture must besuch as to permit imprinting in very thin layers. Thus, for example, theintimate'mixture of finely divided mineral material and water solutionof adhesive should contain, say, at least 50% of solids byweight, and beof printable consistency. If the quantity of adhesive is increased toogreatly at the expense of the mineral matter, the printing consistencyof the coating mixture will be altered, the coating will assume adifferent imprinted pattern; and the desirable matte surface of thefinished board will be impaired if the ratio of adhesive to mineralmatter becomes too high.

A useful decrease in the absorptivity of the coating to the vehicles ofgloss printing inks may be attained by increasing the ratio from, say,15 parts by weight of adhesive to 100 parts by Weight of mineral matterto 20 parts of adhesive to 100 parts mineral matter. The ink vehiclesreferred to may include varnishes, resins, chlorinated rubber,nitrocellulose, etc. The excess of the adhesive reduces porosity,increases cohesion and, under some circumstances, promotes the adhesionof the coating to the Web without significantly altering the desiredappearance of the finished product. But at the same time the coatingbecomes harder, and because it still contains a relatively very largequantity of the mineral matter as compared with the adhesive, a newphenomenon presents a problem. The coating becomes more abrasive andwhen the coated surface of the finished paper or board is rubbed againstitself, or against some other surface, particles of the mineral mattertend to abrade off. This is undesirable in itself and can seriouslyinterfere with printing, and with the final use of the coated article ininstances Where exposed surfaces are rubbed together, as for example,cartons packed together in a shipping container whereby they rubtogether in transit. This phenomenon is generally known as chalking andit may occur where the ratio of adhesive to minerals is low or high. Inthe former instance it is due to a deficiency of adhesive to bind themineral particles together, and in the latter instance to hardness ofthe coated surface with consequent increase in abrading power. Then,too, the intrinsic strength of a given adhesive is significantregardless of the ratio of adhesive to minerals.

I We have found that the chalking difficulty can be corrected throughthe use, in the way hereinafter outlined, of a water soluble materialhaving the characteristics of a wax. The waxy characteristics of such amaterial appear to act in some fashion as a lubricant to diminishfriction when the product is rubbed or abraded, and the tough nature ofthe film formed also tends to prevent the loosening of chalk-likeparticles under abrasion. There are various so-called water solublewaxes which may be employed. Generally these are of the class ofpolymerized ethylene glycols, referred to hereinafter as polyethyleneglycol. Suitable materials are those sold under the trade name Carbowaxby Carbide and Carbon Chemical Company.

Althoughthese waxes are water soluble in nature, they do notsignificantly affect the waterproofness of the surface of the coatedweb. They are preferably not applied as a constituent of the coatingcomposition, but rather as a film in a finishing process. In thepreferred practice of our invention the board is first plasticized asdescribed, then coated by imprinting during a persistence oftheplasticizing effect. It is usual then to pass the board over a smallnumber of drying rolls and finally through a finishing stack of calenderrolls. The water soluble wax, while it may be otherwise applied, ispreferably imposed upon the coated web surface by means of one or morewater boxes on the finishing calender stack. This insures the impositionof an exceedingly thin and controlled film, as will be understood by theskilled worker familiar with water box application.

the manufacture of imprinted coated board by the procedure generallyoutlined above, insolubility is frequently desired in the adhesive usedas a constituent of the mineral coating composition. Thus where caseinor other protein is employed as the adhesive of the mineral coating,'in'solubility is attained by the use of an agent such as. alum whichreacts with the casein to render it insoluble. It is not ordinarilydesired to mix an insolubilizing agent with the coating compositionitself since the reaction will proceed to an undesired extent prior tothe imposition of the coating and will interfere with manufacturingoperations. As a consequence, the alum is ordinarily imposed upon thesurface of the coated web by means of water boxes on the finishingcalender stack. This works out well in the practice of our process. Thealum, with or without other hardening constituents, is mixed with thewater solution of the polyethylene glycol in proper proportions forimposition by means of the water boxes on the finishing calender stack.

Where the adhesive is starch and insolubility is desired, variousprocedures may be adopted. Insolubility in a starch-mineral coatingcomposition can be attained through the use of a suitable melamine,urea, or other fesi'n and a catalyst. Again, it is not desirable toincorporate both the resin and the catalyst in the coating compositionsince a premature hardening will take place. In one satisfactoryprocedure, we incorporate the resin with the plasticizing materials inthe Water solution applied by means of the water boxes on the initialcalender stack mentioned above. The catalyst may then be incorporated inthe starch-mineral coating, and the water soluble wax may be employed onthe finishing stack of calenders. Again, resin may be incorporated withthe starch-mineral coating composition and a catalyst such, for example,as ammonium chloride may be applied together with water soluble wax onthe finishing calender stack.

For the purposes of this invention, we have found no substitute for theuse of the Water soluble wax as a finishing material.Carboxymethylcellulose or other soluble cellulosic compounds may beemployed in the water boxes of a finishing calender stack; but whilethese may afiect to a certain extent the permeability of the coating,they do not have the lubricating effect which has been described andhence do not cure the chalking. Moreover, the water soluble waxes whichhave been de scribed produce a tough film which not only binds themineral particles together at the surface of the coated product, butalso is resistant to the non-water vehicles of inks. Emulsions ordispersions of substances readily compatible with the ink vehicles arenot suitable since they tend to increase the penetration of the mineralbase by the vehicles of inks. It is a characteristic of the watersoluble waxes that, when applied as taught, they cure the chalkingdifficulty without significantly affecting the appearance of the coatedproduct. Paperboard or boxboard is used in very large quantities for theformation of cartons and the like involving the gluing of portions ofthe board together in a lapped joint. It is desired, therefore, to gainthe advantages set forth above with a material which will neitheradversely affect the aflinity of the coated surface of the board forwater vehicle glues or the like but which also will leave the coatedsurface of the board sufficiently penetrable by water to permit a rapidsetting of the glue. These functions are admirably served by the watersoluble waxes, whereas materials having a lubricating action butincompatible with water would seriously interfere with gluing.

By a combination of the three factors which have been described, namelya thorough plasticization of the surface of the board or paper, the useof a somewhat increased ratio of adhesive to mineral matter in thecoating and the-finishing of the product with a thin and uniformapplication of the water soluble wax, we attain a greatly improvedprinting quality and entirely satisfactory imperviousness to the glossvehicles of glossy printing inks while retaining the essentialappearance of the coated product made in the ways described.

Example 1 calender stack. The solution desirably has a low intrinsicviscosity and to this end a polyvinyl alcohol was chosen which is soldunder the trade name Elvanol 7005 by the E. I du Pont de Nemours & Co.This polyvinyl alcohol exhibits a viscosity of 4 to 6 centipoises in a4% water solution by the Hoeppler falling ball method. The solution wasmade up by agitating the polyvinyl alcohol'in water with a suitablemechanical mixing device and heating to about 180 F. until the polyvinylalcohol was completely dissolved. The freshly made board was passedthrough a 9-roll breaker calender stack and the polyvinyl alcoholsolution was applied to the surface of the board to be coated by meansof a water box on the fourth calender roll, the remaining rolls at theirrespective nips mechanically working the surface of the board in thepresence of the polyvinyl alcohol. Additional applications may be madeif desired by means of additional water boxes. The

web surface is roughest at the first roll nip and will absorb more ofthe polyvinyl alcohol solution there, which is desirable from thestandpoint of plasticization, but mechanically the moving web performsbetter on the stack if treated at a lower or later nip. At later nips inthe breaker calender stack, an application of polyvinyl alcohol solutionproduces a greater coating action with less total penetration. Thelimiting factor in plasticizing the surface of the web to be coated isthe standard weight-thickness values recognized in the industry; and thestrength of the polyvinyl alcohol solution, while it may be varied, islargely dependent on the factors of cost and viscosity. Too great aviscosity of polyvinyl alcohol will require a lower content of it in thecalender solution to preventthe solution from being so thick and stickythat it will stick to and pull fibers from the paper surface and thusdetract from the plasticization of the paper surface. The temperature ofthe polyvinyl alcohol solution in the calender box ranged between 140and 170 F. The temperature may be varied through rather wide limits, butan elevated temperature is generally desirable for the plasticizingaction. The application in theexemplary. instance .was carried on bymeans of one water box and was of the order of gallons of the polyvinylalcohol solution per ton.

Instead of polyvinyl alcohol, it is possible to employ otherplasticizing materials. Modified or converted starch,carboxymethylcellulose or any of the other soluble celluloses may beemployed either alone or in admixture with polyvinyl alcohol. Alginic orproteinaceous substances may also be used alone or in admixture with anyof the other substances mentioned.

A coating composition was prepared containing parts of casein by weightin water solution to 100 parts of finely divided mineral coatingmaterial, the latter being dispersed and suspended in the caseinsolution, with the help of a complex phosphate as a dispersing agent tothe extent of producing a coating composition containing from 50% to 55%total solids and being of imprinting consistency. Soda ash was added toassist in the dissolving of the casein. Among other materials suitablefor this purpose are ammonia and borax. A lubricating agent such as soapmay be added if desired in small quantities to assist in the imprintingand subsequent calendering operation. A small amount of coloring mattermay be added to tint the mixture to a desired cast of white. The mineralcoating material was in this instance made up of equal parts of titaniumdioxide and clay. Since the coating is applied in an exceedingly thinlayer, a high opacity coating material is desirable.

The coating was applied by an imprinting roll as part of a coating trainof rolls such as illustrated and described in the said U. S. Patent2,419,207. The thickness of the applied coating was approximately onequarter of one one-thousandth of an inch representing an application ofabout two pounds of coating solids per 1,000 square feet of the boardsurface.

The board was then sent over a sufficient number of drying rolls to dryit and was passed to a 9-roll finishing calender stack. A solution wasmade up containing 4% dry alum and 4% Carbowax solids in water. The alumemployed was regular paperm akers dry, pulverized alum of 17% A1203content and the Carbowax employed was grade PF45 of 50% soluble waxsolids. The alum and Carbowax were agitated in water with air untilcompletely dissolved. Live steam may be used for this purpose ifdesired, but the treatment should not be continued after completesolution. In the exemplary instance one pint of formalin (40%formaldehyde in water) was added to each 100 gallons of the solution.The solution was applied to the mineral coated surface on a water box onthe final calender stack at the temperature as made; a regulation of thetemperature of the solution in the water box at optimum conditions ishelpful in maintaining uniformity of its calendered effect.

The resultant board was a smoothly coated sheet having an attractivewhitematte surface, waterproof in the sense that it was free of wetrubbing tendencies, free of chalking tendency and having superiorprinting quality as respects printing inks in general but especially asto gloss printing inks, which dried thereon with a markedly greaterdegree of gloss.

Example 2 A freshly made boxboard was plasticized as set forth aboveand, during a persistenceof the plasticizing effect, was coated byimprinting with a coating composition containing 23 parts by weight ofstarch as an adhesive to parts by weight of mineral coating material asset forth above, the coating composition also containing sufficientmelamine or urea resin to give resistance to wet rubbing when the resinis polymerized. A small amount of coloring matter may be used to achievethe desired cast of white. The coating was accomplished as set forthabove, the thickness of the coating being the same, and the coatingwasthen dried.

The coated board Waspassed to a finishing stack of calenders and theretreated with a solution containing 4% Carbowax solidsand about 4%ammonium chloride to catalyze the resin, the remainder of the solutionbeing water.

The result again was a board having a smooth, attractive, opaque, mattecoating resistant to wet rubbing, resistant to chalking and havingsuperior printing characteristics.

As an alternative procedure, the resin may beincorporated in theplasticizing solution on the initial calender stack and the ammoniumchloride incorporated in thecoating composition, the treating solutionon the final calender stack being a solution of the water soluble wax inwater without other additives.

in another procedure, the catalyst may be incorporated in theplasticizing solution on the initial calender stack and the resinincorporated in the coating composition, the treating solution on thefinal calender stack being a solution of the water soluble wax in waterwithout other additives.

The quantity of the water soluble wax in the final treatment solutionmay be varied taking into account cost, viscosity and the generaldesirability of avoiding the application of so great a quantity of theWax solids as materially to affect the appearance of the coated web andits ability to receive printing inks. Ordinarily we prefer to employ asolution containing substantially 2% to 7% of the water soluble wax. Itis a characteristic of the solutions taught herein for use on thefinishing calender stack that they will not cause sticking or picking ofthe coated surface thereon even in those instances where the agentrelied upon to impart non-wet-rubbing characteristics to the board isincorporated in the final treatment solution.

Modifications may be made in our invention without departing from thespirit of it. Having thus described our invention in certain exemplaryembodiments, what we claim as new and desire to secure by Letters Patentis:

1. A process of making coated paper or board webs which comprisesplasticizing a surface of said webs, during the persistence of theplasticizing effect imprinting thereon a mineral-adhesive coatingcomposition, drying the webs, passing the webs through a finishingcalender stack and imposing on the coated surfaces of said webs asolution of a water soluble wax by means of at least one water box onthe said finishing calender stack.

2. The process claimed in claim 1 wherein the coating compositionemployed is one suitable for imprinting and contains substantially 20parts by weight of adhesive to 100 parts by weight of mineral coatingsubstance, the

water soluble wax applied during the finishing step serving to lubricateand to increase the toughness of the mineral coated surface.

3. The process claimed in claim 2 wherein the said solution of watersoluble Wax contains also an agent acting to insol-ubilize the saidadhesive in the mineral coating composition.

. 4. The process claimed in claim 2 wherein the said coating compositioncontains casein as the adhesive and the said solution of water solublewax contains alum as a hardening agent for said casein.

5. The process claimed in claim 2 wherein the coating composition afterimprinting contains a resin and starch as'the adhesive, and the saidsolution of water soluble wax contains also a catalyst for said'resin.

6. The process claimed in claim 2 wherein a resin is employed in asolution used in the said plasticizing step and wherein the said coatingcomposition is a composition in which "starch is the adhesive andcontains also a Catalyst for said resin.

-7.' A process of producing a matte coated web which is resistant topenetration by the gloss vehicles of glossy printing inks which processcomprisespassing a paper or paperboard'web through a breaker stack ofcalendars while applying to at least one of its surfaces a plasticizingsolution comprising a material chosen from a class consisting ofpolyvinyl alcohol, soluble cellulose, starch and proteinaceous andalginic substances and mixtures thereof to the extent of thoroughlyplasticizing the said surface, and during a persistence of theplasticizing efiect imprinting upon the said surface an adhesive-mineralcoating composition of suitable consistency for imprinting and to athickness of approximately 0.00025 in., the said coating compositioncontaining at least around 20 parts by weight of adhesive to 100 partsby weight of mineral coating substance, drying the imprinted board andpassing it through afinishing caln-der stack while applying to itssurface a Water solution of polyethylene glycol.

8. .The process claimed in claim 7. wherein the coating compositioncontains casein as the adhesive and wherein the solution of watersoluble wax containsalso a hardening agent for said casein.

9. The process claimed in claim 7 wherein the imprinted coatingcontainsstarch as an adhesive, 21 melamine resin as'an insolubilizing agent, theprocess including the step of catalyzing the said resin. I

10. The process claimed in claim 7 wherein the imprinted coatingcontains starch as an adhesive and a urea resin as an insolubilizingagent, the process including the step of catalyzing said resin.

11. A fibrous web coatedby imprinting and having a matte surfacesubstantially free of chalking tendency and having improved printingcharacteristics, said coated web bearing a coating containing at leastabout 20 parts by weight of adhesive to parts by weight of mineralmatter and carrying a thin, uniform, film-like application of watersoluble wax.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Synthetic Organic Chemicals, 12th ed., Carbide & CarbonChemicals Corp., 1947. (Only pp. 19-23 relied upon.)

1. A PROCESS OF MAKING COATED PAPER OR BOARD WEBS WHICH COMPRISESPLASTICIZING A SURFACE OF SAID WEBS, DURING THE PERSISTENCE OF THEPLASTICIZING EFFECT IMPRINTING THEREON A MINERAL-ADHESIVE COATINGCOMPOSITION, DRYING THE WEBS, PASSING THE WEBS THROUGH A FINISHINGCALENDER STACK AND IMPOSING ON THE COATED SURFACES OF SAID WEBS ASOLUTION OF A WATER SOLUBLE WAX BY MEANS OF AT LEAST ONE WATER BOX ONTHE SAID FINISHING CALENDER STACK.